The present invention relates generally to internal combustion engines, and more particularly, to such an engine having a breather chamber for venting of crankcase gases.
The crankcase of an air-cooled internal combustion engine is sealed from the combustion chamber by the piston rings. These engines sometimes have a crankcase breather system for venting "blow-by" gases which get by the piston rings and valve stem seals and enter the crankcase. The breather system vents these gases to maintain a negative crankcase pressure so that lubrication oil is not forced pass the oil seals by the internal crankcase pressure. Such a breather system often involves a check valve which allows gases to exit the crankcase when the piston is moving toward its bottom dead center position and will close to prevent air from entering the crankcase.
Conventional breather valves include a reed-type valve in which a flexible metal strip is positioned over an opening in the crankcase. The reed opens on the downward piston stroke when the pressure differential between the atmosphere and the crankcase is sufficient to overcome the spring tension of the reed. A second type of valve is a disc-type valve that is generally made of a lightweight metal or plastic and moves perpendicular to an opening in the crankcase. The disc is spring biased or gravity biased to a closed position and opens to cover the opening in response to pressure changes. A third type of valve is a molded flexible valve, commonly called a "mushroom" or "umbrella" valve. This valve has a center stem for attachment to a valve seat and covers a circular pattern of holes in the closed position. When crankcase pressure builds, the valve will flex upwardly to uncover the openings and vent the gases.
In one method of installing a flexible breather valve to a crankcase, a molded plastic cup is provided and sits in an opening in the crankcase. The cup includes a molded flange at the top of the cup and the necessary valve openings in the bottom of the cup. A gasket and metal cover is used to retain the cup/valve assembly and to seal the cup, which is held in place by several screws.
Another method of installing a flexible breather valve includes machining the necessary holes into the wall of the cylinder itself and assembling the breather valve directly to the cylinder. However, two problems arise with this method. First, the surface necessary for proper valve installation and sealing requires machining of the cylinder casting at additional cost. Second, the metal cylinder wall provides a direct path for the transfer of engine heat to the valve, thus promoting valve degradation. In addition, the small openings required for valve function are relatively difficult to cast in place, and would probably have to be machined.
A further method of installing a breather valve is to press and secure the valve seat into an opening in the cylinder block. If the seat is composed of metal, a press fit between the seat and cylinder block should be sufficient for retention and sealing. However, the metal seat provides a direct path for engine heat during engine operation, thereby promoting degradation of the valve retaining stem. Such a problem may be overcome by utilizing a plastic seat; however, the high temperature environment quickly causes the press fit between the seat and the cylinder to disappear due to plastic creep, thereby necessitating the use of some type of mechanical retention, such as staking the surrounding metal over the seat at additional cost.
It is desired to provide an improved flexible breather valve assembly which can be securely and reliably installed and retained without the use of separate fasteners or fastening methods.